1. Field of the Invention
The present invention relates to a semiconductor device designed to operate by using two-dimensional electron gas as channel, an electric circuit using such a semiconductor device and a method of controlling such an electric circuit.
2. Description of the Related Art
HEMT (high electron mobility transistor) is known as typical semiconductor device that employs two-dimensional electron gas (2DEG) as channel for operation. In a HEMT, 2DEG is formed along the interface of a channel layer (non-doped GaN, for example) and an electron supply layer (AlGaN, for example) at the side of the channel layer. A channel is formed in the 2DEG between the source and the drain of the transistor and turned on and off by the gate of the transistor for switching operations.
Particularly, GaN-based HEMT, where GaN is a nitride semiconductor, is suited for switching operations for high power because the bandgap of GaN is large and the electron saturation velocity is high. Therefore, a switching circuit that operates with high power can be formed by using GaN-based HEMT. FIG. 6 illustrates an exemplar full bridge circuit (electric circuit) for driving a motor formed by using four HEMTs as switching devices. With the illustrated circuit configuration, two arms, each having two switching devices connected in series between a high side terminal and a low side terminal are formed in juxtaposition. The connection point of the two switching devices of one of the arms and that of the two switching devices of the other arm are respectively denoted by A and B and the opposite terminals of coil L for driving a motor are connected to them. The direction in which an electric current flows through the coil L is controlled by switching the high side transistors (Tr1, Tr2) and the low side transistors (Tr3, Tr4). For example, an electric current flows by way of the route indicated by an arrowed broken line in FIG. 6 when Tr1 and Tr4 are turned on while Tr2 and Tr3 are turned off. An electric current flows in the opposite direction when Tr1 and Tr4 are turned off while Tr2 and Tr3 are turned on. In this way, the sense of rotation and stop of a motor can be controlled by controlling each of the transistors Tr1 through Tr4 for on and off. Particularly, such a circuit can be used for controlling high power at high speed when HEMTs are used as Tr1 through Tr4.
When transistors are employed to operate for high power, monitoring the operations of the transistors is an effective for security. For example, the electric current flowing through Tr1 may be monitored before Tr4 is turned on, to determine whether Tr4 can be turned on properly by the monitored electric current value. With such an arrangement, a motor will be safely operated. Patent Document 1 (Jpn. Pat. Appln. Laid-Open Publication No. 2006-108679) describes such a current detection type HEMT. With the technique described in Patent Document 1, a plurality of sets of a source electrode, a gate electrode and a drain electrode of HEMT are arranged in array and an electrode region for current detection is formed at the outside. The electrode region operates as pseudo source electrode and the drain current flowing to HEMT can be monitored by observing the electric current flowing to the electrode region.
HEMT that can detect an excess current is obtained by using this technique. Then, a switching circuit that operates safely with high power is obtained by using such HEMT.
However, when detecting an electric current by means of a current detection device, electric power is consumed in this current detection device in addition to the power consumed for the intrinsic operation of HEMT. It has been difficult to monitor the operation of HEMT in an on-chip manner without an increase of power consumption.